A common hermetic terminal used for a crystal oscillator or the like includes a base made of a Kovar material (an alloy containing 54% of iron, 28% of nickel, and 18% of cobalt), a lead also made of a Kovar material, an insulating glass which seals the base and the lead, and an iron cap into which the base is press-fitted and fixed. A pair of through holes are formed in the base, and the lead penetrates each through hole. A gap between the lead and the base is hermetically sealed with the insulating glass. The lead and the base are electrically insulated by the insulating glass.
In such a conventional hermetic terminal, in order to ensure solderability of an outer lead when mounting a printed circuit board, solder alloy plating or electrolytic plating such as tin plating, nickel plating, gold plating, or the like is performed on the entire surface of the hermetic terminal.
As a method of electrolytic plating, a barrel plating method is adopted. In the barrel plating method, a large number of hermetic terminals are housed in a barrel having liquid permeability, and the barrel is immersed in a plating bath. The immersed barrel is rotated to plate the large number of hermetic terminals at once.
Further, as a method for selectively performing solder alloy plating, nickel plating, gold plating, silver plating, rhodium plating, or the like on a base and a lead, a method described in PTD 1 is adopted, for example.
Here, passive components constituting an electronic circuit include an aluminum electrolytic capacitor. The aluminum electrolytic capacitor includes a high-purity anode aluminum foil, a cathode aluminum foil, an electrolytic solution, and capacitor paper. The high-purity anode aluminum foil has an oxide film formed on a surface which serves as a dielectric.
The aluminum electrolytic capacitor has a capacitor element formed by causing the anode foil and the cathode foil to face each other, sandwiching the capacitor paper therebetween, and cylindrically winding them. However, in this state, the capacitor element has a small electrostatic capacitance. By impregnating the capacitor paper with the electrolytic solution to serve as electrolytic paper, an anode foil surface and a cathode foil surface are electrically connected, and thus a capacitor element having a large electrostatic capacitance which uses an aluminum oxide film on the anode foil surface as a dielectric is obtained (see NPD 1).
This electrolytic solution acts as a real cathode, and the aluminum electrolytic capacitor ends its life when the electrolytic solution dries up. Since the aluminum electrolytic capacitor generally belongs to a group of components having the shortest life among the components constituting an electronic circuit, there has recently been a demand for an aluminum electrolytic capacitor having a longer life.
In recent electronic circuits, an electronic component is often mounted in a narrow gap. Also in aluminum electrolytic capacitors, in addition to conventional aluminum electrolytic capacitors of the cylindrical case type, aluminum electrolytic capacitors having a non-cylindrical shape, such as those having the shape of a flat plate with a low height, have increased. In these aluminum electrolytic capacitors having a non-cylindrical shape, a bottom portion of a case has the shape of a rectangle or an ellipse. In a conventional aluminum electrolytic capacitor of the cylindrical case type, a disk-shaped rubber packing is inserted into a cylindrical case, and sealing is performed by uniformly swaging an end portion of the case. When the bottom portion of the case has the shape of a rectangle or an ellipse, it is difficult to perform sealing by uniform swaging.
In order to achieve an aluminum electrolytic capacitor having a longer life, it is favorable if the aforementioned hermetic terminal can be used to seal a case which houses a capacitor element. By using the hermetic terminal, hermeticity is improved, and drying-up of an electrolytic solution can be prevented. In addition, since press fitting and resistance welding can be used when sealing the hermetic terminal to the case, hermeticity can be ensured without being influenced by the shape of the case.
However, in a conventional hermetic terminal, a base material made of iron or an iron-based alloy is subjected to electroplating with a soft metal, such as solder plating, tin plating, nickel plating, gold plating, or the like. When these conventional plating films are brought into contact with an electrolytic solution for a long period of time, the plating metal or the metal composing the base material is gradually dissolved in the electrolytic solution and contaminates the electrolytic solution. Since contamination of the electrolytic solution has an adverse effect on the properties of a capacitor, it is not possible to use the conventional hermetic terminal for an aluminum electrolytic capacitor.
Although it is also conceivable to use aluminum to compose the base and the lead of the hermetic terminal, it has not been possible to realize this conception, for reasons that there is no glass material whose thermal expansion coefficient matches that of aluminum, that the step of sealing with glass in the hermetic terminal requires heating to about 1,000° C. and results in melting of an aluminum material, and the like.